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Mol. Cell. Biol., Mar 1996, 818-828, Vol 16, No. 3
D Yan and M Ares Jr
U2 small nuclear RNA (snRNA) contains a sequence (GUAGUA) that pairs with
the intron branchpoint during splicing. This sequence is contained within a
longer invariant sequence of unknown secondary structure and function that
extends between U2 and I and stem IIa. A part of this region has been
proposed to pair with U6 in a structure called helix III. We made mutations
to test the function of these nucleotides in yeast U2 snRNA. Most single
base changes cause no obvious growth defects; however, several single and
double mutations are lethal or conditional lethal and cause a block before
the first step of splicing. We used U6 compensatory mutations to assess the
contribution of helix III and found that if it forms, helix III is
dispensable for splicing in Saccharomyces cerevisiae. On the other hand,
mutations in known protein components of the splicing apparatus suppress or
enhance the phenotypes of mutations within the invariant sequence that
connect the branchpoint recognition sequence to stem IIa. Lethal mutations
in the region are suppressed by Cus1-54p, a mutant yeast splicing factor
homologous to a mammalian SF3b subunit. Synthetic lethal interactions show
that this region collaborates with the DEAD-box protein Prp5p and the yeast
SF3a subunits Prp9p, Prp11p, and Prp21p. Together, the data show that the
highly conserved RNA element downstream of the branchpoint recognition
sequence of U2 snRNA in yeast cells functions primarily with the proteins
that make up SF3 rather than with U6 snRNA.
Copyright © 1996, American Society for Microbiology
Invariant U2 RNA sequences bordering the branchpoint recognition region are essential for interaction with yeast SF3a and SF3b subunits [published erratum appears in Mol Cell Biol 1996 Jul;16(7):3980]
Biology Department, Sinsheimer Laboratories, University of California, Santa Cruz 95064, USA.
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